Flexographic element and method of imaging

ABSTRACT

A relief (or flexographic) printing precursor has first and second radiation-sensitive layers, or a plurality of radiation-sensitive layers. The first radiation-sensitive layer is sensitive to a first imaging radiation having a first λ max . The second radiation-sensitive layer is disposed on the first radiation-sensitive layer and is sensitive to a second imaging radiation having a second λ max  that differs from the first λ max  by at least 25 nm. An infrared radiation ablatable layer can be present and is opaque or insensitive to the first and second imaging radiations and contains an infrared radiation absorbing compound. These relief printing precursors can be used to prepare flexographic printing plates, cylinders, or sleeves where the ablatable layer is used to form an integral mask on the element. Use of the invention provides a relief image without any loss in the strength of the small dots and can be carried out using multiple irradiation steps using the same apparatus.

FIELD OF THE INVENTION

This invention is directed to flexographic elements (or relief printingelements or precursors) that can be used in imaging methods to provideflexographic printing elements with relief images.

BACKGROUND OF THE INVENTION

Flexographic printing is a method of direct rotary printing that uses aresilient relief image in a plate of rubber or photopolymer to printarticles such as cartons, bags, labels, or books. Flexographic printinghas found particular application in packaging, where it has displacedphotogravure and offset lithography printing techniques in many cases.

Many methods of forming relief images are known in the graphic arts.Generally, photosensitive elements comprising an ablatable mask layerover a photosensitive polymer layer can be made into articles bearingrelief images without the use of a photographic negative (graphic artsfilm) or other separate masking device. These photosensitive elementsare formed into relief images by first imagewise exposing the elementwith laser radiation (generally infrared radiation directed by acomputer) to selectively remove the mask layer in the exposed areas, andthen overall exposing the element with actinic radiation (typically UVradiation) to cure the photosensitive layer in the non-masked areas. Theremaining areas of the mask layer and the non-hardened portions of thephotosensitive layer are then removed by one or more liquid developingprocesses.

Examples of flexographic printing precursors are described for examplein U.S. Pat. No. 5,262,275 (Fan), U.S. Pat. No. 5,703,310 (Van Zoeren),U.S. Pat. No. 5,719,009 (Fan), U.S. Pat. No. 6,020,108 (Goffing et al.),U.S. Pat. No. 6,037,102 (Loerzer et al.), U.S. Pat. No. 6,238,837 (Fan),and U.S. Pat. No. 6,759,175 (Daems et al.) and in EP 0295818 (Cusdin).

Commercial flexographic printing precursors can be prepared from KodakFlexcel® NX Plate that is used with Kodak Flexcel® NX Thermal media andthat is from Eastman Kodak Company and Cyrel® Digital Flexo Plate thatis available from DuPont.

U.S. Pat. No. 7,279,254 (Zwadlo) describes a method for making anarticle with a relief image using a removable film.

Other flexographic printing elements are described in U.S. PatentApplication Publication 2005/0227182 (Ali et al.) and in copending andcommonly assigned U.S. Ser. No. 11/758,042 (filed Jun. 5, 2007 byZwadlo, Brown, Fohrenkamm, and Stolt) that describes a masking film andmethod of using it to improve the relief image.

Flexographic printing elements having integral mask layers (that is,ablatable layers) generally require the use of a high-poweredlaser-equipped imaging that is specifically configured for imaging suchelements. In some instances, multiple machines may be needed to vary thethickness of the relief image.

While the quality of articles printed using flexographic elements hasimproved significantly in recent years, physical limitations related tothe process of creating a relief image in the flexographic printingplate remain. For example, it is very difficult to print small graphicelements such as fine dots, lines, and even text using flexographicprinting elements. The density of the image is represented by the totalarea of the various-sized dots in a halftone screen representation of acontinuous tone image. In the lightest areas of an image (commonlyreferred to as highlights), the dots need to become very small. In thetraditional flexographic imaging process, the small dots are generallylimited to 4%. Due to the nature of the plate making processes,maintaining small dots on a flexographic printing plate is verydifficult. In a pre-imaging (or post-imaging) step, the floor of therelief image is set by area exposure to ultraviolet light from the backof the printing element. This exposure hardens the photopolymer to adesired relief depth for optimal printing. Floodwise exposure toimage-forming radiation via a mask layer followed by a processing stepto remove unhardened (that is, unexposed) photopolymer produces reliefdots having generally conical shape.

The smallest of these dots are sometimes removed during processing,which means that no ink is transferred to those areas during printing(the dot is not “held” on the plate or press). Alternatively, even ifthe smallest dots survive processing, they are susceptible to damage onthe rotary printer, as small dots often fold over or partially break offduring printing causing either excess ink or no ink to be transferredduring printing.

Copending and commonly assigned U.S. Ser. No. 12/183,173 (filed Jul. 31,2008 by Zwadlo) describes making a relief image on a flexographicprinting plate using selective backside exposure to curing radiation.This allows the formation of highlight dots down to 0.8%.

SUMMARY OF THE INVENTION

This invention provides a relief printing precursor comprising at least:

a first radiation-sensitive layer that is sensitive to a first imagingradiation comprising a first λ_(max),

a second radiation-sensitive layer disposed on the firstradiation-sensitive layer, the second radiation-sensitive layer beingsensitive to a second imaging radiation comprising a second λ_(max) thatdiffers from the first λ_(max) by at least 25 nm, and

an infrared radiation ablatable layer disposed over the secondradiation-sensitive layer, the ablatable layer being opaque to the firstand second imaging radiations and comprising an infrared radiationabsorbing compound.

In some embodiments of this invention, a relief printing precursorcomprises two or more radiation-sensitive layers comprising, at least:

a first radiation-sensitive layer that is sensitive to a first imagingradiation comprising a first λ_(max), and

a second radiation-sensitive layer disposed on the firstradiation-sensitive layer, the second radiation-sensitive layer beingsensitive to a second imaging radiation comprising a second λ_(max) thatdiffers from the first λ_(max) by at least 25 nm.

This invention also provides a method of making a relief printing imagecomprising the steps of:

A) imagewise exposing a relief printing precursor described above toinfrared radiation ablative energy to form a mask image in the infraredradiation ablatable layer,

B) subsequently or simultaneously with step A, exposing the reliefprinting precursor to a second imaging radiation through the mask image,and

C) modifying the mask image using additional infrared radiation ablativeenergy to form a modified mask image.

In some embodiments, the method includes after step C, an additionalstep:

D) subsequently exposing the relief printing precursor to a firstimaging radiation through the modified mask image.

Further, this invention provides a relief printing system comprising:

a) a first unit comprising a first radiation-sensitive layer that issensitive to a first imaging radiation comprising a first λ_(max), and

b) a second unit comprising a second radiation-sensitive layer that issensitive to a second imaging radiation comprising a second λ_(max) thatdiffers from the first λ_(max) by at least 25 nm, and an infraredradiation ablatable layer disposed on the second radiation-sensitivelayer, the ablatable layer being opaque to the first and second imagingradiations and comprising an infrared radiation absorbing compound.

This invention also provides a relief printing precursor comprising:

two or more radiation-sensitive layers having different radiationsensitivities, and

an infrared radiation ablatable layer that is opaque to imagingradiation to which the two or more radiation-sensitive layers areresponsive.

In some embodiments, these precursors are self-supporting and one of theradiation-sensitive layers is capable of being cured to provide asubstrate.

In other embodiments, the precursor further comprises a transparentsubstrate and the infrared radiation ablatable layer is on the oppositeside of the substrate from the two or more radiation-sensitive layers.

Still again, a method of making a relief printing image comprises thefollowing steps:

A′. imagewise exposing the relief printing precursor described above toinfrared radiation ablative energy to form or modify a mask image in theinfrared radiation ablatable layer, and

B.′ exposing the relief printing precursor through the mask image usingradiation to which at least one of the radiation-sensitive layers issensitive,

Steps A′ and B′ can repeated in sequence at least once, and in someembodiments, each B′ step can be carried out using differentwavelengths.

In addition, this invention provides a relief printing precursorcomprising two or more radiation-sensitive layers comprising, at least:

a first radiation-sensitive layer that is sensitive to a first imagingradiation comprising a first λ_(max), and

a second radiation-sensitive layer disposed on the firstradiation-sensitive layer, the second radiation-sensitive layer beingsensitive to a second imaging radiation comprising a second λ_(max) thatdiffers from said first λ_(max) by at least 25 nm.

This relief printing precursor can further comprise an infraredradiation ablatable layer disposed on the second radiation-sensitivelayer, the infrared radiation ablatable layer being opaque to the firstand second imaging radiations and comprising an infrared radiationabsorbing compound.

Applicants have discovered a way to prepare a relief image without anyloss in the strength of the small dots in that relief image. With thepresent invention, they can selectively control the relief image floorusing a second frontside IR ablative exposure instead of a backsideexposure using the same mask and exposing machine, leaving a perfectlyregistered relief image. In other words, a single mask layer can be usedto define a relief image and floor pattern (that is multi-level pattern)by imaging from the same side using the same machine. This avoids theneed to create a second mask for backside imaging. When backsideexposure is used in combination with frontside exposure, a secondexposing device is needed along with possibly two masks. The presentinvention eliminates the need for a backside mask and exposureapparatus. The relief printing precursor of this invention can be eitherpositive- or negative-working and the substrate can be transparent oropaque since a backside exposure is not needed.

These advantages are achieved by including a second infrared radiationablative exposure from the frontside to expand the mask for the reliefimage that was initially created with a first IR ablative exposure. Inaddition, the relief image is formed using two separate layers thatdiffer in sensitivity by at least 25 nm. The present invention enableshigher throughput for individual imaging apparatus and only a singleapparatus is needed. Current commercial imaging machines, such as theKodak® Trendsetter image setting device can be readily modified to carryout the present invention.

Because the relief printing precursor of this invention has two or morelayers of different radiation sensitivity, the resulting relief imagehas two or more levels corresponding to the number ofradiation-sensitive layers. Thus, each level in the relief image has adifferent composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view, not to scale, of a reliefprinting precursor of this invention, showing its various layers.

FIG. 2 is a partial cross-sectional view, not to scale, of theembodiment of FIG. 1 after the mask has been initially formed by laserablation in the infrared radiation ablatable layer.

FIG. 3A is a partial cross-sectional view, not to scale, of theembodiment of FIG. 2 after exposure to the second imaging radiation andoptionally exposure to the first imaging radiation at the same time.

FIG. 3B is a partial cross-sectional view, not to scale, of theembodiment of FIG. 2 after exposure to the second imaging radiationonly.

FIG. 4 is a partial cross-sectional view, not to scale, of theembodiment of FIGS. 3A and 3B after a second laser ablation step toenlarge the mask.

FIG. 5 is a partial cross-sectional view, not to scale, of theembodiment of FIG. 4 after exposure to the first imaging radiation toenlarge the relief floor.

FIGS. 6A and 6B are partial cross-sectional views, not to scale, ofembodiments of FIG. 5 after chemical development to removenon-crosslinked portions of the layers and to provide relief images.

FIGS. 7A and 7B are partial cross-sectional views, not to scale, of anembodiment of a relief printing system of this invention having firstand second units that can be laminated to form a relief printingprecursor.

FIG. 7C is a partial cross-sectional view, not to scale, of theembodiments of FIGS. 7A and 7B after chemical development to removenon-crosslinked portions of the layers and to provide a relief image.

FIGS. 8A, 8B, 9A, 9B, and 10A are partial cross-sectional views, not toscale, of a method of the prior art for providing a relief printingelement.

FIG. 10B is a partial cross-sectional view, not to scale, of a reliefprinting image of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Unless otherwise indicated, the relief printing precursor describedherein is an embodiment of this invention, and can be in the form of asheet, plate, cylinder, sleeve, tape, web, or any other shape that canbe used for providing a relief image.

Unless otherwise indicated, all percentages are by weight.

By “ablative” or “infrared radiation ablatable layer”, we mean that thelayer can be imaged using a thermal ablative means such as IR laserradiation that causes rapid local changes in the imageable layer therebycausing the layer material(s) to be vaporized or ejected from thatlayer.

Useful embodiments of the relief printing precursor and a method for itsuse can be understood by reference to FIGS. 1-7, and further detailsabout compositions of layers are provided below.

Referring to FIG. 1, relief printing precursor 10 includes substrate 15having disposed thereon first radiation-sensitive layer 20, secondradiation-sensitive layer 25, infrared radiation ablatable layer 30, andoptional oxygen inhibition layer 35.

Referring to FIG. 2, relief printing precursor 10 can be irradiated withinfrared radiation ablative energy (not shown), for example, using aninfrared radiation laser that is commercially available, to provide anintegral mask so that image areas 45 are removed from infrared radiationablatable layer 30. The integral mask can be formed, for example, usinga Kodak® Trendsetter plate setter (Eastman Kodak Company, Vancouver,Calif.). During formation of the mask, little or no changes occur in theunderlying layers. This laser ablative step can be carried out with thenoted apparatus or other known apparatus, for example, at an exposureenergy of at least 150 and up to 2000 mJ/cm². Exposure is usuallycarried out for at least 2 and up to 6 minutes. The image can besupplied by digital signals from a suitable computer that has thedesired image stored in suitable digital form.

FIGS. 3A and 3B show the effect of irradiation of relief printingprecursor 10 with a second imaging radiation (λ₂) that includesradiation that is typically in the UV to visible region of theelectromagnetic spectrum (for example, from about 250 to about 750 nm orUV only of from about 290 to about 320 nm). This irradiation can occursimultaneously with or subsequently to the laser ablation operationdescribed above. In some embodiments, the second imaging radiation is inthe near-IR to IR regions of from about 750 to about 1400 nm ortypically from about 750 to about 1200 nm. The secondradiation-sensitive layer 25 is sensitive to this second imagingradiation that crosslinks, “cures”, coalesces, or otherwise hardens theirradiated portions 50 of that layer. This irradiation can be providedfrom any suitable source such as a fluorescent lamp, JDSU Diode Laser(830 nm) Fiber-Coupled 2.0 W (2486-L3 Series), or Nichia UV LED modelNCSU034A(T) (385 nm), or Nichia UV LED model NCSU033A(T) (365 nm), orRoithner UV LED model UVTOP255-FW-TO39 (255 nm). Roithner has 255-380 nmUV LEDs available and Phoseon Technology's RX Starfire MAX UV LightSystem using UV LED Array 380-420 nm can be used. Philips TL-80W is aUVA bulb radiating at 365 nm, and Philips TUV-36W is a UVC bulbradiating at 254 nm.

Optionally, flexographic precursor 10 (FIG. 3A) can be simultaneously orsequentially irradiated with a first imaging radiation (λ₁) to whichsecond radiation-sensitive layer is insensitive but to which firstradiation-sensitive layer 20 is sensitive to provide crosslinked,“cured”, coalesced, or otherwise hardened regions 55. Thus, theirradiation of these layers may be over a broad range of wavelengths(such as a broadband lamp) so that both first and second imagingradiation (λ₁+λ₂) occur at the same time. In some embodiments, such anirradiation source can be fitted into an existing IR-imaging apparatusso that the same apparatus can be used for both ablation and irradiationof the underlying layers. For example, a Kodak® Trendsetter plate settercan be modified in this manner.

Optionally, relief printing precursor 10 may contain first radiationsensitive layer 20 that is sensitized to both the first imagingradiation λ₁ and second imaging radiation λ₂ such that the step ofexposing the second radiation layer 25 to the second imaging radiationλ₂ creates crosslinked, cured, coalesced, or otherwise hardenedirradiated portions 50 (that will be inked during printing) and alsoprovides crosslinking or curing of irradiated portions 55.

FIG. 3B shows the option of irradiating relief printing precursor 10with second imaging radiation λ₂ only

This imaging of the second radiation-sensitive layer can be carried outusing the noted imaging radiation source for at exposure energy of atleast 100 mJ/cm² and up to and including 5 J/cm². The time of exposurewould be readily determined depending upon the particular apparatus andirradiation sources being used. As noted above, this irradiation canoccur at the same time as the laser ablation step and thus, the time forboth steps would be the same.

FIG. 4 shows that a second laser ablation of infrared radiationablatable layer 30 in relief printing precursor 10 provides a modifiedor expanded mask, for example in image area 45′. The user can beselective as to the parts of the mask to be modified, for example,choosing to modify only those image areas in the mask that will providesmaller image dots. Alternatively, the user may modify only those areasin the mask adjacent to edges of the image where there are no adjacenthalf-tone dots. One skilled in the art would readily recognize otherareas in the element where it would be desirable to locally raise thefloor of features in the printing plate (for example, features may behalf-tone dots, linework, text, solid areas, solid printed areas or anyother imageable area). Ablative energy for this step can be at least 150mJ/cm² and up to 2 J/cm² for at least 2 minutes and up to 6 minutes.

Once the second laser ablation is carried out, relief printing precursor10 can be irradiated with first imaging radiation (λ₁) to furthercrosslink, cure, coalesce, or otherwise harden first radiation-sensitivelayer 20 to expand the cured regions 55, and to form new regions 55′(see FIG. 5). As noted above, in some embodiments, there may be noprevious curing of first radiation-sensitive layer 20, and this may bethe first time in the process that it is irradiated with curingradiation that differs from the second imaging radiation by at least 25nm. For example, the first imaging radiation can be generally in the UVto visible range of from about 250 to about 750 nm or typically fromabout 290 to about 400 nm. This irradiation step is used to strengthenor expand the “floor” of the eventual relief image by curing more offirst radiation-sensitive layer 20 in new regions 55′ through modifiedmask area 45′. The first imaging radiation can be carried out for atleast 1 and up to 20 minutes (for example from about 4 to about 20minutes) at exposure energy of at least 2 J/cm² and up to and including50 J/cm².

If desired, a backside exposure (not shown) can be carried out at thispoint using the first radiation through substrate 15 to raise the reliefimage floor. This exposure can be, for example, for 20 seconds at 376mJ/cm².

In many embodiments, the second radiation-sensitive layer is opaque,transparent, or insensitive to the first imaging radiation. The firstand second imaging radiations can have overlapping wavelengths as longas their λ_(max) values differ by at least 25 nm. In other embodiments,the λ_(max) values are different but the first and second imagingradiations do not have overlapping wavelengths. The firstradiation-sensitive layer must have sensitivity to radiation to whichthe second radiation-sensitive layer is not sensitive.

In FIG. 6A, the non-imaged portions of the first and secondradiation-sensitive layers and the optional oxygen inhibition layer havebeen washed off in chemical processing or development to provide reliefprinting element 10′, having a relief image shown by raised image areas65 and local floor 60 on substrate 15. It should be understood however,that such elements need not have a substrate but can be self supportingif the first radiation-sensitive layer is thick enough. This chemicalwashing or development can be carried out using the appropriate solventsthat dissolve the non-imaged portions of the noted layers.

Alternatively, in FIG. 6B, relief image floor 70 of relief printingelement 10′ has been raised using an optional backside exposure of thefirst radiation-sensitive layer.

This invention also provides a relief printing assembly that isillustrated in FIGS. 7A and 7B. Relief printing assembly 100 comprisesfirst unit 110 that comprises first radiation-sensitive layer 115 thatis sensitive to a first imaging radiation comprising a first Am asdescribed above and optional substrate 125. Second unit 120 includessecond radiation-sensitive layer 140, laser ablatable layer 135 in whicha modified mask has already been formed with removed areas 142 and 142′and optional oxygen inhibition layer 130. The mask need not be alreadyformed in every relief printing assembly of this invention (for example,ablation or mask formation can be accomplished after the two units arejoined). In FIG. 7A, second radiation-sensitive layer 140 has alreadybeen irradiated with a second imaging radiation as described above toprovide areas 150. In such embodiments, steps A and B of the method ofthis invention are carried out before the first and second units areunited (which is also before step D of the method). Either or both thefirst and second units can be self-supporting but in the embodimentillustrated in FIG. 7A, first unit 110 has substrate 125. Optionalpeelable protective layer 145 can also be present to protect second unit120 and can also serve as a substrate that is removed or peeled offafter laser ablation and irradiation and before the two units are joinedor united by lamination or other suitable means. For example, laminationcan be carried out using a Kodak Flexel NX® Laminator (Eastman KodakCompany, Rochester, N.Y.) or a CODOR LLLP650 Laminator (Codor LaminatingSystems, Amsterdam, Holland). Supports or substrates are optional foreach of the first and second units shown in FIG. 7A.

For example, in FIG. 7A, second unit 120 can be irradiated with infraredradiation ablative energy (not shown) to provide a mask so that imagedareas 142 of infrared radiation ablatable layer 135 are removed. Thismask formation can be accomplished, for example using a Kodak®Trendsetter plate setter. During formation of this mask, little or nochange occurs in the underlying layers. They are opaque or insensitiveto this ablative exposure. Second radiation-sensitive layer 140 isirradiated with a second imaging radiation (not shown) comprising λ₂creating crosslinked or cured regions 150. Then, second unit 120 isfurther exposed to infrared radiation ablative energy (not shown) tomodify the integral mask so some of image areas 142 are enlarged toimage area 142′. An optional protective layer 145 is peeled off andthen, second unit 120 is laminated to first unit 110 to form integralrelief printing precursor 100. As shown in FIG. 7B, first imagingradiation comprising λ₁ is then applied to first radiation-sensitivelayer 115 trough the modified mask areas 142 and 142′ in infraredradiation ablatable layer 135 to create enlarged crosslinked or curedregions 155 and 155′ in first radiation-sensitive layer 115. A reliefimage with raised image areas 65 and local floor 60 on substrate 125 isprovided in relief printing element 100′ by chemical development (FIG.7C).

An optional back exposure step may then be performed by exposing firstradiation-sensitive layer 115 to the first imaging radiation λ₁ from aUV source through substrate 125 to provide raised floor 70 in reliefprinting element 100′ as shown after chemical development in FIG. 7D.

Referring to FIGS. 8A, 8B, 9A, and 9B, embodiments of prior art reliefprinting precursors include substrate 15 having thereon singleradiation-sensitive layer 20, oxygen inhibition layer 35 and laserablatable layer 30. A backside exposure to radiation (λ) is used tocreate relief image floor 20′ in radiation-sensitive layer 20. Laserablation is then used to create a mask by providing image areas 45 inlaser ablatable layer 30 (FIG. 9A). Imaging radiation through the mask(FIG. 9B) then provide irradiated portions 50. After chemicaldevelopment, prior art relief printing element 1 is provided (FIG. 10A)that is shown in contrast with relief printing element 10′ (FIG. 10B) ofthe present invention that has an improved relief floor for the reliefimage.

While the embodiments of the present invention that are shown in FIGS. 1through 7D are illustrated in the form of a flexographic printing plateprecursor used to form a flexographic printing plate, the same type ofelements and method steps can be used to provide flexographic printingsleeves to be put onto printing cylinders that serve as substrates, orto form flexographic printing cylinders that are coated with thenecessary layers on a cylinder base that serves as a substrate. Thus,the relief printing precursors can be flexographic printing plate,sleeve, or cylinder precursors. For example, when the relief imageprecursor is in cylindrical form, the appropriate coatings can be laserablated and irradiation using multiple imaging sources as the cylinderis turned at a suitable speed for the given imaging energies andapparatus. When the relief printing precursor is a printing cylinder,this invention provides the advantage that a printing element with anopaque substrate can be imaged using multiple irradiation sources in thesame imaging apparatus.

The relief printing precursor may include a suitable dimensionallystable substrate upon which the radiation-sensitive layers and ablatablelayer are disposed, and optionally a separation layer, oxygen barrierlayer, cover sheet, or metal layer. Suitable substrates include but arenot limited to, dimensionally stable polymeric films, such as polyester,polyolefin, acrylic, polycarbonates, polyamides, and cellulose acetatefilms known in the art, and metals such as aluminum sheets, sleeves, orcylinders. Since backside irradiation is not necessary, the substratecan also be opaque and include various papers or pigmented resins. Eachor both of the first and second units used for the relief printingsystem can have a suitable substrate but in most embodiments, only thefirst unit has a substrate.

While the present invention is illustrated primarily with respect torelief printing precursors having two radiation-sensitive layers, itwould be readily apparent to one skilled in the art from this teachingand the precursors could have two or more radiation-sensitive layers aslong as the individual layers have sensitivity that different from theothers by at least 25 nm.

The first and second radiation-sensitive layers are designed to haveradiation-sensitive compositions that are sensitive to specificelectromagnetic radiation. This sensitivity can cause various chemicalor mechanical changes such as polymerization, crosslinking, curing,coalescence, chain scission and decomposition. As pointed out above, thefirst radiation-sensitive layer is generally sensitive to radiationhaving a first λ_(max) of from about 250 to about 750 nm or from about290 to about 400 nm. This is accomplished by incorporating appropriatesensitizing compounds into that layer. The second radiation-sensitivelayer can be generally sensitive to radiation having a second λ_(max) inthe range of from about 250 to about 750 nm or from about 290 to about320 nm, as long as the first and second λ_(max) values are at least 25nm apart. In other embodiments, the second λ_(max) is in the range offrom about 750 to about 1400 nm.

In general, each of the radiation-sensitive layers includes a curableradiation-sensitive composition (for example a UV-curable composition)that generally photopolymerizable, photocrosslinkable, or both. Thesecompositions generally include one or more curable resins (such as aUV-curable resin) or pre-polymer or one or more polymerizable monomers,one or more photoinitiators, and one or more elastomeric resins. Thesecompositions are curable upon exposure to the selected irradiation andthe non-cured composition is soluble or dispersible in a suitabledeveloping solvent(s). The compositions can include various addenda thatare known in the art including but not limited to, plasticizers,rheology modifiers, thermal polymerization inhibitors, tackifiers,colorants, antioxidants, antioxonants, and fillers. Variousradiation-sensitive compositions and components thereof are describedfor example in U.S. Pat. No. 6,238,837 (Fan) that is incorporated hereinby reference and in references cited therein.

Examples of useful elastomeric binders include but are not limited to,natural or synthetic polymers of conjugated diolefin hydrocarbonsincluding polyisoprene, polybutadienes, butadiene/acrylonitrile,butadiene/styrene thermoplastic-elastomeric block copolymers, and othercopolymers known in the art for this purpose. The elastomeric binder(s)can be present in either or both of the first and secondradiation-sensitive layers in the same or different amounts of at least50 weight %.

One or more crosslinkable components in the radiation-sensitivecompositions include but are not limited to, ethylenically unsaturatedpolymerizable compounds (monomers or oligomers) having a molecularweight of less than 30,000 such as (meth)acrylates, di(meth)acrylates,pentaeryiritol di- and tri-acrylates, (meth)acrylate derivatives ofisocyanates, esters, and epoxides, and others that are known in the art.It is also possible to use crosslinkable polymers such as those havingfree radical reactive pendant or side groups for example as described inU.S. Pat. No. 5,840,463 (Blanchet-Fincher). The monomer(s) is present inthe first and second radiation-sensitive layers in the same or differentamounts of at least 5 weight %.

The photoinitiators are compounds that are sensitive to the specificradiation and generate free-radicals that initiate polymerization of themonomer(s) without excessive termination. In many embodiments, thephotoinitiators are sensitive to UV or visible radiation and should bethermally inactive at and below 185° C. Examples of usefulphotoinitiators of this type include but are not limited to peroxides(such as benzoyl peroxide), azo compounds (such as2,2′-azobis(butyronitrilie)), benzoin derivatives (such as benzoin andbenzoin methyl ether), derivatives of acetophenone (such as2,2-dimethoxy-2-phenylacetophenone), ketoxime esters of benzoin,substituted and unsubstituted polynuclear quinones, triazines,3-ketocoumarins, and biimidazoles. In other embodiments, thephotoinitiators in the second radiation-sensitive layer are near-IR orIR sensitizers or catalysts including but not limited to onium salts.The photoinitiator(s) is generally present in the first and secondradiation-sensitive layers in the same or different amounts of at least0.001 weight % and typically from about 0.1 weight % to about 10 weight%.

The thickness ratio of the first radiation-sensitive layer to the secondradiation-sensitive layer is at least 1:1 and up to 500:1 and typicallyfrom about 50:1 to about 300:1. Thus, the first radiation-sensitivelayer generally has a thickness of from about 1000 to about 3000 μm, andthe second radiation-sensitive layer generally has a thickness of fromabout 10 to about 150 μm, and typically from about 50 to about 75 μm.

The infrared radiation ablatable layer in the relief printing precursoris ablatable (vaporizes or decomposes) upon exposure to infraredradiation. The layer generally includes one or more compounds capable ofabsorbing near-infrared and infrared radiation of from about 750 toabout 1400 nm and typically from about 800 to about 1250 nm, and one ormore binders. This layer provides a “mask” for imaging the underlyinglayers and is thus opaque to the imaging radiation and has atransmission optical density of 2 or more and typically of 3 or more.The absorption feature is provided by incorporating one or more infraredradiation-absorbing compounds into the layer. Such compounds include butare not limited to carbon black and other organic or organic pigments,and infrared-absorbing dyes such as cyanine, squarylium,chalcogenopyrloarylidene, polymethine, oxyindolizine, merocyanine, metalthiolate, and quinoid dyes. These absorbing compounds can be present inany concentration that is effective for the intended purpose andtypically from about 0.1 to about 30 weight %.

Useful binders for the infrared radiation ablatable layer include butare not limited to, polymers that are incompatible with underlyinglayers and are generally tack-free, such as polyamides, polyvinylacetals, polyimides, polybutadienes, silicone resins, polycarbonates,polyesters, polyalkylenes, polylactones, and polyacetals. The binder(s)can be present in an amount of from about 40 to about 90 weight % andtypically from about 60 to about 80 weight %.

The infrared radiation ablatable layer can also include one or moreplasticizers, pigment dispersants, surfactants, adhesion modifiers,coating aids, and secondary binders such as polystyrenes, polyacrylates,polyvinylidene chloride, polyurethanes, and polyvinyl chloride.

The infrared radiation-ablatable layer generally has a thickness of lessthan 1 μm.

In some embodiments, an oxygen inhibition layer can be disposed betweenthe second radiation-sensitive layer and the infrared radiationablatable layer. This oxygen barrier layer can shield the underlyingradiation-sensitive layers from atmospheric oxygen and minimizemigration of materials out of those layers into the infrared radiationablatable layer. Materials useful in such layers include but are notlimited to, poly(vinylidene chloride), poly(vinyl alcohol)s, andstyrene-maleic anhydride copolymers.

As noted above, curing of the first and second radiation-sensitivelayers can be carried out using suitable UV or visible (or IR) imagingsources. For example, UV or visible imaging sources include carbon arcs,mercury vapor arcs, fluorescent lamps, electron flash units, sun lamps,and photographic flood lamps. IR imaging sources include laser diodesand thermal resistive heads.

Imagewise exposure to provide the mask and irradiation of theradiation-sensitive layers can be accomplished using the same ordifferent equipment, for example using a drum upon which the reliefprinting precursor is mounted and rotated to allow for exposure to thedifferent radiations.

IR ablation time can vary as described above depending upon thethickness of the infrared radiation ablatable layer, the complexity ofthe image, the distance from the precursor, and the nature andcomposition of the ablatable layer.

As described above, actinic radiation (UV to visible) exposure time canvary from a few seconds to a few minutes depending upon the intensityand spectral energy distribution of the radiation, its distance from theprecursor, and the nature and amount of curable compositions in the tworadiation-sensitive layers.

After all of the irradiation steps, the relief image is completed bydeveloping or removing the non-imaged portions of the layers using adeveloper than dissolves, disperses, or swells the non-imaged portionsso they can be removed. Suitable developers include organic solventssuch as aliphatic or aromatic hydrocarbons (especially non-chlorinatedhydrocarbons), long chain alcohols, or mixtures thereof. Some developersmay also include some water or alkaline components. Commercialdevelopers include those sold by DuPont as CYREL OptiSol and CYRELCyloSol developers. Examples of solvents used in developers are alsodescribed for example in U.S. Pat. No. 3,782,961 (Takahashi et al.),U.S. Pat. No. 4,517,279 (Worns), and U.S. Pat. No. 4,847,182 (Worns etal.), U.S. Pat. No. 5,354,645 (Schober et al.), U.S. Pat. No. 3,796,602(Briney et al.), and DE 3,828,551. Solution development can beaccompanied by mechanical removal means such as scrubbing, rubbing,wiping, or brushing means. Development may require a time of at least 2and up to 20 minutes at from about 20° C. to about 35° C. and thesolution can be applied by immersion, dipping, spraying, brushes, orrollers. The type of developing apparatus and specific developer thatare used will dictate the specific development conditions.

If the non-imaged portions of the mask are not removable duringdevelopment, a pre-development step may be used to remove those portionsfirst, for example by using an etching solution or highly alkalinesolution.

Following development, the resulting relief printing elements aregenerally blotted or wiped dry, and possibly dried in a forced air orinfrared oven using conventional drying times and temperatures.Detackification (or light finishing for example using λ₁ and λ₂) is anoptional post-development treatment that can be applied if the printingsurface is still tacky. For embodiments having more than tworadiation-sensitive layers, the detackification can be carried out usingany or all of the wavelengths to which the layers are sensitive. Apost-development curing may also be carried out if desired.

The resulting relief printing elements can be used to advantage in theformation of seamless, continuous flexographic printing elements. Flatsheets can be wrapped around a cylindrical form, usually as a printingsleeve or the printing cylinder itself and joining or taping the endstogether. However, as noted above, the method of this invention can beperformed while the precursor is mounted around a cylindrical form.

The resulting relief image can have a total depth of up to 2000 μm thatcan represent up to 100% of the original thickness of both first andsecond radiation-sensitive layers if the precursor comprises asubstrate. If the precursor is “self-supporting” and has no separatesubstrate, the total depth may represent up to 60% of the originalthickness of both of the radiation-sensitive layers.

The following embodiments represent some ways that the present inventioncould be designed and used:

Embodiment 1

The first radiation-sensitive layer could contain Esacure KTO 46photoinitiator at a concentration of 0.2 g/l making it sensitive to UVradiation between 360 to 400 nm. The second radiation-sensitive layercould contain Esacure KB1 at 0.01 g/l making it sensitive to UVradiation below 320 nm. The IR ablation layer could have the compositiondescribed in Table 1 of Example 1 of U.S. Pat. No. 7,279,254 (Zwadlo etal.) that is incorporated herein by reference.

Embodiment 2

The first radiation-sensitive layer could contain Esacure KTO 46photoinitiator at a concentration around 0.2 g/l making it sensitive toUV radiation between 360 to 400 nm. The second radiation-sensitive layercould contain Esacure KTO 46 at a concentration around 0.01 g/l makingit sensitive to UV radiation below 340 nm. The IR ablation layer couldhave the composition described in Example 1 of U.S. Pat. No. 7,279,254(noted above).

Embodiment 3

The first radiation-sensitive layer could contain Esacure KTO 46Photoinitiator at a concentration around 0.2 g/l making it sensitive toUV radiation between 360 to 400 nm. The second-radiation sensitive layercould contain Spectra Group H-Nu-IR-780 photoinitiator with peaksensitivity near 765 nm. The IR ablation layer could have thecomposition described in Table 1 of Example 1 of U.S. Pat. No. 7,279,254(noted above).

Embodiment 4

The first radiation-sensitive layer precursor 110 could have a KodakFlexcel® NXH Plate sensitive to UV radiation up to 380 nm. The secondunit 120 could contain Spectra Group H-Nu-IR-780 photoinitiator withpeak sensitivity near 765 nm. Second unit 120 also could contain the IRablation layer 135 having the composition of Table 1 of Example 1 ofU.S. Pat. No. 7,279,254 (noted above). A Kodak® Trendsetter NX platesetter having a laser wavelength of 830 nm can be used to ablate the IRablation layer to form a mask. A 765 nm light source can be used insidethe plate setter to expose the second radiation-sensitive layer. TheKodak® Trendsetter NX plate setter can then be used to image the IRlayer and expand mask in areas where the local floor is desired. Secondunit 120 could then be laminated to first unit 110 using a Kodak NXLaminator. Resulting precursor 100 can then be exposed using a Mekromexposure unit using UV energy below 400 nm. The IR ablation layer can bepeeled off and precursor 100 less the IR layer is solvent washed outusing a Mekrom processor.

Embodiment 5

The first radiation-sensitive layer precursor 110 can have a KodakFlexcel® NXH Plate sensitive to UV radiation up to 380 nm. Second unit120 can contain second radiation sensitive layer that could containEsacure KTO 46 at a concentration around 0.01 g/l making it sensitive toUV radiation below 340 nm. Second unit 120 can also contain IR ablationlayer 135 having the composition described in Table 1 of Example 1 ofU.S. Pat. No. 7,279,254 (noted above). A Kodak Trendsetter NX with alaser wavelength of 830 nm is used to ablate the IR layer. A UV lightsource with radiation contain up 350 nm light is used inside theTrendsetter to expose the second radiation-sensitive layer. A Kodak®Trendsetter NX can then be used to image the IR layer and expand areaswhere the local floor is desired. Second unit 120 can be laminated tofirst unit 110 using a Kodak® NX Laminator. First unit 100 can then beexposed using a Mekrom exposure unit using UV energy above 360 nm andbelow 400 nm. The IR ablation layer can be peeled apart and precursor100 less the IR layer can be solvent washed out using a Mekromprocessor.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

1. A relief printing precursor comprising: two or more radiation-sensitive layers having different radiation sensitivities, and an infrared radiation ablatable layer, said ablatable layer being opaque to imaging radiation to which said two or more radiation-sensitive layers are responsive.
 2. The precursor of claim 1 that is self-supporting and wherein one of said radiation-sensitive layers is capable of being cured to provide a substrate.
 3. The precursor of claim 1 further comprising a transparent substrate wherein said infrared radiation ablatable layer is on the opposite side of said substrate from said two or more radiation-sensitive layers.
 4. A relief printing precursor comprising at least: a first radiation-sensitive layer that is sensitive to a first imaging radiation comprising a first λ_(max), a second radiation-sensitive layer disposed on said first radiation-sensitive layer, said second radiation-sensitive layer being sensitive to a second imaging radiation comprising a second λ_(max) that differs from said first λ_(max) by at least 25 nm, and an infrared radiation ablatable layer disposed on said second radiation-sensitive layer, said ablatable layer being opaque to said first and second imaging radiations and comprising an infrared radiation absorbing compound.
 5. The relief printing precursor of claim 4 that is a flexographic printing precursor that is in the form of a flexographic printing plate, sleeve, or cylinder precursor.
 6. The relief printing precursor of claim 4 wherein said first λ_(max) is from about 250 to about 750 nm, and said second λ_(max) is from about 250 to about 750 nm.
 7. The relief printing precursor of claim 4 wherein said first λ_(max) is from about 250 to about 750 nm, and said second λ_(max) is from about 750 to about 1400 nm.
 8. The relief printing precursor of claim 4 wherein said first λ_(max) is from about 290 nm to about 400 nm, and said second λ_(max) is from about 290 to about 320 nm.
 9. The relief printing precursor of claim 4 wherein said second radiation-sensitive layer is also transparent to said first imaging radiation.
 10. The relief printing precursor of claim 4 that is a flexographic printing sleeve precursor.
 11. The relief printing precursor of claim 4 said first radiation-sensitive layer is disposed on a substrate.
 12. The relief printing precursor of claim 4 that is a coated flexographic printing cylinder precursor.
 13. The relief printing precursor of claim 4 that is a self-supporting flexographic printing plate precursor.
 14. The relief printing precursor of claim 4 wherein the thickness ratio of said first radiation-sensitive layer to said second radiation-sensitive layer is at least 1:1 and up to 300:1.
 15. The relief printing precursor of claim 4 wherein said first radiation-sensitive layer and said second radiation-sensitive layer comprise a UV-curable composition comprising a UV-curable component, a photoinitiator, and an elastomeric resin.
 16. A relief printing system comprising: a) a first unit comprising a first radiation-sensitive layer that is sensitive to a first imaging radiation comprising a first λ_(max), and b) a second unit comprising a second radiation-sensitive layer that is sensitive to a second imaging radiation comprising a second λ_(max) that differs from said first λ_(max) by at least 25 nm, and an infrared radiation ablatable layer disposed on said second radiation-sensitive layer, said ablatable layer being opaque to said first and second imaging radiations and comprising an infrared radiation absorbing compound.
 17. The relief printing system of claim 16 wherein either or both of said first and second units are self-supporting.
 18. The relief printing system of claim 16 wherein said second unit further comprises a peelable protective layer disposed on said second radiation-sensitive layer.
 19. A method of making a relief printing image comprising the following steps: A) imagewise exposing the relief printing precursor of claim 4 to infrared radiation ablative energy to form a mask image in said infrared radiation ablatable layer, B) subsequently or simultaneously with step A, exposing said relief printing precursor to a second imaging radiation through said mask image, and C) modifying said mask image using additional infrared radiation ablative energy to form a modified mask image.
 20. The method of claim 19 further comprising the step of: D) subsequently exposing said relief printing precursor to a first imaging radiation through said modified mask image.
 21. The method of claim 20 wherein prior to step D, said first radiation-sensitive layer is laminated as a first unit to a second unit comprising said second radiation-sensitive layer and said infrared radiation ablatable layer.
 22. The method of claim 21 wherein said second unit further comprises a peelable protective layer that is peeled off before lamination of said first and second units.
 23. The method of claim 20 wherein after steps A and B, said first radiation-sensitive layer is laminated as a first unit to a second unit comprising said second radiation-sensitive layer and said infrared radiation ablatable layer.
 24. The method of claim 20 wherein said first imaging radiation has a λ_(max) of from about 290 nm to about 400 nm, and said second imaging radiation having a λ_(max) of from about 290 to about 320 nm.
 25. The method of claim 20 wherein said relief printing image is formed on a flexographic printing cylinder or printing sleeve.
 26. The method of claim 20 wherein said relief printing image is formed on a flexographic printing plate.
 27. A relief printing precursor comprising two or more radiation-sensitive layers comprising, at least: a first radiation-sensitive layer that is sensitive to a first imaging radiation comprising a first λ_(max), and a second radiation-sensitive layer disposed on said first radiation-sensitive layer, said second radiation-sensitive layer being sensitive to a second imaging radiation comprising a second λ_(max) that differs from said first λ_(max) by at least 25 nm.
 28. The relief printing precursor of claim 27 further comprising an infrared radiation ablatable layer disposed on said second radiation-sensitive layer, said infrared radiation ablatable layer being opaque to said first and second imaging radiations and comprising an infrared radiation absorbing compound.
 29. A method of making a relief printing image comprising the following steps: A′. imagewise exposing the relief printing precursor of claim 1 to infrared radiation ablative energy to form or modify a mask image in said infrared radiation ablatable layer, B.′ exposing said relief printing precursor through said mask image using radiation to which at least one of said radiation-sensitive layers is sensitive,.
 30. The method of claim 29 wherein steps A′ and B′ are repeated in sequence at least once.
 31. The method of claim 29 wherein each B′ step is carried out using different wavelengths. 